1. Field of the Invention
This invention relates to a backlight device for a transmission-type liquid crystal display, especially of a field sequential system.
2. Description of the Related Art
Recently, colored liquid crystal displays have been widely used for displaying color images in notebook-type personal computers or compact liquid crystal TV monitors.
Except for special rare systems, display of color images on color liquid crystal displays relies on color filters.
In liquid crystal display devices combined with color filters to display color images, three dots of different colors, namely, red, green and blue, are combined to display a desired color. However, color filters are very expensive and need a high accuracy when bonded to panels. Moreover, they need a triple number of dots to ensure an equivalent resolution as compared with black-and-white liquid crystal display panels. Therefore, typical liquid crystal color panels require a triple number of drive circuits in the horizontal direction. This means an increase of the cost of drive circuits themselves and the cost for increased man-hours for connecting the drive circuits to the panel at a triple number of points. Thus, from the economical viewpoint, the use of color filters with liquid crystal panels to display color images involves many disadvantages.
Another problem with color filters is their optical transmittance which can be as low as 20% approximately. When color filters are used, the luminance decreases to approximately one fifth, and a large electric power is consumed for backlighting to compensate for the luminance.
Japanese Patent Laid-Open 1-179914 (1989) discloses a color liquid crystal display device employing a field sequential system to display color images by combining a black-and-white liquid crystal panel and tricolor backlighting instead of using color filters. Certainly, this method appears more likely to realize high-fidelity color images inexpensively. As to tricolor backlighting, Japanese Patent Applications Nos. 7-271994 and 8-49476 propose the use of three cold cathode-ray tubes generating three colors, red, green and blue, which are packaged in a space equivalent to that of a thin-type backlight device using conventional white cold cathode-ray tubes and optical guide plates.
However, an inverter circuit required to generate a high voltage for lighting the cold cathode-ray tubes is difficult to switch by means of an inexpensive semiconductor switch, and it was necessary to connect inverters to respective cold cathode-ray tubes so as to control the lighting of three cold cathode-ray tubes by controlling the outputs of three inverters. Moreover, although the lighting time is reduced to less than one third, the peak current does not change so much. Therefore, coils and other elements cannot be miniaturized, and three inverters of the same size as conventional inverters must be used. This is a serious problem in applications to notebook-type personal computers for which miniaturization is one of most important requirements. Also from the economical viewpoint, inverters are expensive and have as much as several times the cost of cold cathode-ray tubes.
There is another approach as shown in FIG. 3 in which switches 8, 9 and 10 made of bi-directional thyristors, which are readily available, inexpensive, resistant to high voltages, and permit electric current to flow in opposite directions, are connected in series to three cold cathode-ray tubes 5, 6 and 7 generating three colors, red, green and blue, so as to apply a voltage to them from a high-voltage generating means 4a made up of a d.c. power source 1 and an inverter 3.
However, when a voltage is applied from the high-voltage generating means 4a, the resistance to the voltage of these switches 8, 9 and 10 made of the bi-directional thyristors is not sufficient under the condition where all of the switches are turned OFF, which results in an electric discharge in one or more of these cold cathode-ray tubes 5, 6 and 7 and failure to light completely.
Therefore, it has been considered difficult to control the lighting of the cold cathode-ray tubes by bi-directional thyristors, and conventional technologies were compelled to independently connect inverters 3a, 3b and 3c to individual cold cathode-ray tubes 5, 6 and 7 as shown in FIG. 4 and to control the lighting of the cold cathode-ray tubes 5, 6 and 7 by controlling the supplied power source to the respective inverters 3a, 3b and 3c by switches 2a, 2b and 2c. Therefore, the above-indicated problems still remain.